Light trapping optical cover

ABSTRACT

A light trapping optical cover employing an optically transparent layer with a plurality of light deflecting elements. The transparent layer is configured for an unimpeded light passage through its body and has a broad light input surface and an opposing broad light output surface. The light deflecting elements deflect light incident into the transparent layer at a sufficiently high bend angle with respect to a surface normal and direct the deflected light toward a light harvesting device adjacent to the light output surface. The deflected light is retained by means of at least TIR in the system formed by the optical cover and the light harvesting device which allows for longer light propagation paths through the photoabsorptive layer of the device and for an improved light absorption. The optical cover may further employ a focusing array of light collectors being pairwise associated with the respective light deflecting elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 13/345,738,filed 8 Jan. 2012, which is a continuation-in-part of application Ser.No. 12/764,867, filed Apr. 21, 2010. This application also claimspriority from U.S. provisional application Ser. No. 61/214,331 filed onApr. 21, 2009, and U.S. provisional application Ser. No. 61/461,522filed on Jan. 18, 2011, incorporated herein by reference in itsentirety.

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device and method for enhancing thelight trapping in light harvesting devices. Particularly, the presentinvention relates to collecting light from a large surface area of thelight harvesting device comprising a light absorbing material andtrapping the light within the device so as to increase the optical paththrough the light absorbing material and improve the useful lightabsorption. More particularly, the present invention relates toenhancing the light trapping in photovoltaic solar panels, lightdetectors, day lighting systems, bioreactors, water light-treatmentreactors, and the like.

2. Description of Background Art

Many light harvesting devices employ a light-absorbing active layer thathas at least a partial transparency with respect to the incident lightor absorbs more weakly in certain wavelengths than in the others.Conventionally, the absorption in such devices can be improved byincreasing the thickness of the active layer. However, this results inthe increased system dimensions, material consumption, weight and cost.Alternatively, light trapping approaches are well known in which thelight path is altered within the device by micro-texturing one or moredevice surfaces. While this allows to somewhat increase the light pathlength and thus improve absorption compared to a non-textured device, asignificant portion of the light may still escape from the devicewithout being fully absorbed. It is therefore an object of thisinvention to provide an improved optical structure that can be used inconjunction with light harvesting devices and that can provide efficientlight trapping with minimal energy loss.

The present invention solves the above problems by providing atransparent optical cover structure having one or more micro-structuredsurfaces that allow for trapping the incident light within the lightharvesting device by means of at least TIR and cause the multiplepassage of the trapped light through the active layer thus improving thelight absorbtion and device efficiency at the minimum consumption ofactive layer's material. Other objects and advantages of this inventionwill be apparent to those skilled in the art from the followingdisclosure.

BRIEF SUMMARY OF THE INVENTION

The present invention solves a number of light harvesting problemswithin a compact system utilizing efficient light deflection andtrapping mechanisms. An optically transparent layer is provided whichcan be placed on top of a light harvesting device and enhance the usefullight absorption in the device. The transparent layer employs lightdeflecting elements that communicate incident light a sufficiently highbend angle within the layer allowing for TIR at its light input surfaceand increasing the optical path length of light rays through thephotoabsorptive layer of the light harvesting device.

In at least one embodiment, the present invention describes an opticalcover which deflects light at a greater propagation angle with respectto a surface normal and traps said light by means of a total internalreflection which allows for increasing the light path length and formultiple passage of light through the photoabsorptive layer of a lightharvesting device.

The optical cover includes a layer of optically transparent materialhaving a broad light input surface and an opposing broad light outputsurface extending generally parallel to the light input surface. Thetransparent layer is configured for an unimpeded transversal lightpassage in the direction from the light input surface towards the lightoutput surface. The transparent layer includes a plurality of lightdeflecting elements distributed along the prevailing plane of the layerand having a cumulative aperture substantially smaller than the area ofeach of the broad surfaces. The light input surface is characterized bya stepped drop in refractive index outwardly from the transparent layerand by a critical angle of TIR. Each of the light deflecting elements isconfigured to receive light propagating between the input and outputsurfaces and bend the light to a greater propagation angle with respectto a normal to the light input surface. The propagation angle of thedeflected light with respect to the surface normal is advantageouslyselected to be greater than the TIR angle characterizing the light inputsurface.

The optical cover operates in response to light received on the lightinput surface of the optically transparent layer. At least a substantialportion of light received by the apertures of light deflecting elementsis deflected from the original propagation path at a greater propagationangle allowing for TIR from the light input surface. As light enters alight harvesting device adjacent to the light output surface, anunabsorbed portion of light reflecting from the front surface or any ofthe internal layers or surfaces of the light harvesting device isreflected by the light input surface by means of TIR. This effectuatesrecycling of light that cannot be absorbed in a single pass through thephotoabsorptive layer of the light harvesting device.

In at least one implementation, the light deflecting elements comprisesurface relief elements. In at least one implementation, the lightdeflecting elements comprise microscopic surface cavities. In at leastone implementation, such cavities may have a V-shape in a cross-section.

In alternative implementations, the light deflecting elements comprisesurface relief features that can be configured in different ways.Particularly, the surface relief features can selected from the group ofelements consisting of prismatic grooves, blind holes, through holes,undercuts, notches, surface discontinuities, discontinuities in saidlayer, surface texture, and surface corrugations.

In at least one implementation, each of the light deflecting elementscomprises a surface inclined at an angle with respect to the light inputsurface and configured to deflect light by means of refraction or atotal internal reflection. In at least one implementation, the inclinedsurface has a planar shape or profile. In at least one implementation,the inclined surface has a curved shape or profile.

In at least one implementation, the optical cover comprising a pluralityof light collectors distributed along the prevailing plane of thetransparent layer. The light collectors are preferably distributedaccording to the same pattern as the plurality of light deflectingelements and pairwise form individual opticules with the respectivelight deflecting elements. Each light deflecting element is disposed onthe optical axis of the respective light collector and in the immediateproximity to the focal area of the collector within each individualopticule. In at least one implementation, the optical cover comprises alens array including a plurality of surface relief features disposed inthe focal plane of the array.

In at least one implementation, the optical cover comprising a lensarray where each lens of the array has a shape in a longitudinal sectionselected from the group of elements consisting of elongated,cylindrical, square, rectangular and hexagonal.

In at least one implementation, the optical cover comprises one or moreoptical cladding layers.

In at least one implementation, the optical cover further comprises oneor more light harvesting devices disposed along the light outputsurface. In at least one implementation, the light harvesting device isselected from the group of elements consisting of one or morephotovoltaic cells, radiation detectors, light absorbers, photo-chemicalreactors and photo-bioreactors.

In at least one implementation, the optical cover has a form of aflexible sheet or film and can be bent to any suitable shape.

The present invention provides a number of beneficial elements which canbe implemented either separately or in any desired combination withoutdeparting from the present teachings.

An element of the invention is an apparatus for collecting light over agiven area and traveling in a generally transversal direction withrespect to the light collection area.

Another element of the invention is the inclusion of an opticallytransparent layer having opposing light input and output surfaces andconfigured for an unimpeded light passage through its body at least in atransversal direction with respect to the either surface.

Another element of the invention is the inclusion of distributed lightdeflecting elements within the interior of the transparent layer whichincrease the propagation angle with respect to a surface normal withoutreversing the prevailing direction of light propagation through thetransparent layer.

Another element of the invention is the use of light deflecting elementscomprising a face containing both a reflective and transmissive surfacefor redirecting the light in relation to a normal to the prevailingplane of the transparent layer.

Another element of the invention is the use of deflecting elementsformed in either light input or light output surface of the opticallytransparent layer.

Another element of the invention is the use of an array of lightfocusing elements which collect and focus the incident light onto therespective deflecting elements.

Another element of the invention is the use of an array of deflectingand/or focusing elements which span the surface of the device, or aportion thereof.

Another element of the invention is the arrangement of the respectivepairs of the light focusing elements and the light deflecting elementsinto individual opticules which can operate independently from the otheropticules.

Another element of the invention is an optical cover configured with anattached optically responsive device (e.g., photovoltaic cell or photoreactor).

Further elements of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a cross-sectional view of an optical cover according to atleast one embodiment of the present invention;

FIG. 2 is a schematic perspective view of an optical cover portionshowing a light deflecting element shaped as an elongated V-groove,according to at least one embodiment of the present invention;

FIG. 3 shows another example of a light deflecting element comprising aprismatic cavity, according to at least one embodiment of the presentinvention;

FIG. 4 shows a further example of a surface relief feature comprising apyramidal cavity, according to at least one embodiment of the presentinvention;

FIG. 5 shows a yet further example of a surface relief featurecomprising a conical cavity, according to at least one embodiment of thepresent invention;

FIG. 6 is a schematic perspective view of an optical cover comprising aplurality of V-shape prismatic grooves in a cylindrical configuration,according to at least one embodiment of the present invention;

FIG. 7 is a schematic perspective view of an optical cover comprising aplurality of V-shape grooves in an axisymmetrical configuration,according to at least one embodiment of the present invention;

FIG. 8 is a schematic perspective view of an optical cover comprising aplurality of discrete light deflecting elements formed by surfacecavities, according to at least one embodiment of the present invention;

FIG. 9 is a further example of light deflecting elements employinganother-shape cavities, according to at least one embodiment of thepresent invention;

FIG. 10 is a schematic perspective view of an optical cover comprisingcavities or V-grooves and further employing a lens array, according toat least one embodiment of the present invention;

FIG. 11 is a schematic perspective view of an optical cover with adifferent disposition of cavities or V-grooves with respect to a lensarray, according to at least one embodiment of the present invention;

FIG. 12 is a schematic perspective view of a rectangular lens arrayemploying cylindrical (linear-focus) lenses, according to at least oneembodiment of the present invention;

FIG. 13 is a schematic perspective view of a rectangular lens arrayemploying square-shaped point-focus lenses, according to at least oneembodiment of the present invention;

FIG. 14 is a schematic perspective view of a rectangular lens arrayemploying hexagon-shaped point-focus lenses, according to at least oneembodiment of the present invention;

FIG. 15 is a schematic perspective view of an optical cover illustratingan exemplary lenticular configuration employing a planar transparentlayer and a lens array, according to at least one embodiment of thepresent invention;

FIG. 16 is a schematic cross-sectional view of an optical coverillustrating its operation in conjunction with a light harvestingdevice, according to at least one embodiment of the present invention;

FIG. 17 is a schematic view, in a cross-section, and ray tracing of alight harvesting system employing an optical cover, according to atleast one embodiment of the present invention;

FIG. 18 is a schematic view, in a cross-section, and ray tracing of alight harvesting system employing an optical cover in an alternativeexemplary configuration, according to at least one embodiment of thepresent invention;

FIG. 19 is a schematic view, in a cross-section, and ray tracing of asunlight harvesting system employing an optical cover and photovoltaicdevices, according to at least one embodiment of the present invention;

FIG. 20 is another example and raytracing of a sunlight harvestingsystem employing an optical cover and photovoltaic devices, according toat least one embodiment of the present invention;

FIG. 21 is a further example and raytracing of a sunlight harvestingsystem employing an optical cover and liquid-carrying photo reactor,according to at least one embodiment of the present invention;

FIG. 22 is a schematic view, in a cross-section, and ray tracing of a anoptical cover showing microstructures associated with a lens array,according to at least one embodiment of the present invention;

FIG. 23 is a schematic cross-sectional view showing an exemplaryindividual light deflecting element and raytracing, according to atleast one embodiment of the present invention;

FIG. 24 is a schematic view, in a cross-section, illustrating a step inmaking an optical cover, according to at least one embodiment of thepresent invention;

FIG. 25 is a schematic view of an optical cover in a sheet roll form,according to at least one embodiment of the present invention;

FIG. 26A through FIG. 26F illustrate various cross-sections of lightdeflecting elements.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the apparatus generally shown inthe preceding figures. It will be appreciated that the apparatus mayvary as to configuration and as to details of the parts withoutdeparting from the basic concepts as disclosed herein. Furthermore,elements represented in one embodiment as taught herein are applicablewithout limitation to other embodiments taught herein, and incombination with those embodiments and what is known in the art.

A wide range of applications exist for the present invention in relationto the collection of electromagnetic radiant energy, such as light, in abroad spectrum or any suitable spectral bands or domains. Therefore, forthe sake of simplicity of expression, without limiting generality ofthis invention, the term “light” will be used herein although thegeneral terms “electromagnetic energy”, “electromagnetic radiation”,“radiant energy” or exemplary terms like “visible light”, “infraredlight”, or “ultraviolet light” would also be appropriate.

FIG. 1 illustrates the present invention and shows a cross-sectionalview of an embodiment of a light trapping optical cover 2. Optical cover2 comprises a layer 8 of essentially transparent refractive materialconfined between broad surface 10 and an opposing broad surface 12. Bothsurfaces 11 and 12 are broadly extending both longitudinally andlaterally so that the thickness of transparent layer 8 is substantiallysmaller compared to its other two dimensions.

Both surfaces 10 and 12 are also essentially smooth and transparent andare configured for a good optical transmission in either direction.Layer 8 is configured for a generally unimpeded light passage throughits body in either direction. Particularly, layer 8 should allow for anunimpeded light passage of light through any parts of the layer in thetransversal direction. Layer 8 should also be sufficiently transparentand allow light to travel considerable distances within the layer alongthe layer's prevailing plane.

Optical cover 2 is generally designed to be laid flat on top of a lightharvesting device (not shown in FIG. 1) where either one of surfaces 10and 12 can be designated to be a light input surface facing the lightsource while the other surface can be designated to be a light outputsurface facing the light harvesting device. In operation, cover 2 andthe underlying light harvesting device may be positioned with theirprevailing surface planes perpendicular to the light source direction.However, they may also be designed to operate at any angle other thannormal.

The refractive material of layer 8 should be high enough so that whenoptical cover 2 is coupled to a light harvesting device, the light inputsurface of layer 8 can form an optical interface characterized by astepped drop in refractive index outwardly from said layer. It will beappreciated by those skilled in the art of optics that when referring tolight or other waves passing through a boundary formed between twodifferent refractive media, such as air and glass, the ratio of thesines of the angles of incidence and of refraction is a constant thatdepends on the ratio of refractive indices of the media. Referring tothe refractive medium of layer 8 and the outside medium immediatelyadjacent to the light input surface, it will be appreciated that thefollowing relationship can describe light bending properties of theoptical interface formed by the light input surface: n₁ sin φ₁=n₂ sinφ₂, where n₁ and n₂ are the refractive indices of the material of layer8 and the outside medium, respectively, and φ₁ and φ₂ are the respectivepropagation angles that light makes in respect to the surface normal. Itwill be further appreciated that, in respect to the light internallystriking the light input surface from layer 8, the same opticalinterface can also be characterized by the angle of a Total InternalReflection (TIR) which is the value of φ₂ for which φ₁ equals 90°. A TIRangle φ_(TIR) can be found from the following expression:

φ_(TIR)=arcsin(n₂/n₁·sin 90°)=arcsin(n₂/n₁). In an exemplary case of theinterface between glass with the reflective index n₁ of about 1.51 andair with n₂ of about 1, φ_(TIR) is approximately equal to 41.47°.

Layer 8 comprises a plurality of light deflecting elements 14 within theboundaries formed by surfaces 10 and 12. Light deflecting elements 14are spaced apart from each other and distributed along the prevailingplane of cover 2. Each light deflecting element 14 has a substantiallysmaller aperture than the light receiving aperture of optical cover 2.Furthermore, the aperture of each light deflecting element 14 ispreferably smaller than the adjacent spacing area so that the pluralityof light deflecting elements 14 cumulatively occupies a sufficientlysmall area compared to either surfaces 10 and 12.

According to an aspect of the present invention, it is preferred thateach light deflecting element 14 is configured to communicate agenerally greater bend angle to light propagating between surfaces 10and 12 compared to the case when light passes through layer 8 simply bycrossing surfaces 10 and 12 and without striking any light deflectingelement 14. Each light deflecting element 14 should preferably beconfigured to alter the ordinary light path between surface 10 and 12yet providing for an unimpeded passage of incident light through layer8.

By way of example and not limitation, light deflecting elements 14 maybe configured to receive light incident onto the light input surface oflayer 8 at normal angles (which corresponds to zero incidence angleswith respect to a surface normal) and deflect it at an angle greaterthan TIR angle φ_(TIR) with respect to the surface normal. In a furthernon-limiting example, each light deflecting element 14 may be configuredto receive a fan of rays having a predefined angular spread and deflecteach ray from the original propagation path so that at least asubstantial part of deflected light rays continues propagating throughlayer 8 but at generally greater propagation angles with respect to anormal to the prevailing plane of the layer. Similarly, it may bepreferred that the new propagation angles, after deflection, are greaterthan TIR angle φ_(TIR) at the optical interface formed by the lightinput surface of layer 8. Accordingly, when surface 10 is designated asthe light input surface, at least a substantial portion of lightdeflected by each element 14 should be communicated a propagation anglegreater than the TIR angle at the boundary formed by surface 10. Whensurface 12 is the light input surface, the propagation angle of thedeflected light should be generally greater than the TIR angle at theboundary formed by that surface.

Let's define a propagation angle φ_(D) being the angle that a light raymakes with respect to a normal to the prevailing plane of layer 8 and,consequently, of optical cover 2. Let's further define angle φ_(D) asbeing counted off from a reference direction along said normal whichindicates the prevailing direction of light propagation through opticalcover 2. For example, when surface 10 of is the light input surface andsurface 12 is the light output surface of layer 8, the prevailingpropagation direction will be the direction from surface 10 to surface12 along the surface normal. Likewise, when surface 12 is receivinglight and surface 10 is the opposing light output surface, theprevailing direction of light propagation through cover 2 will be thedirection from surface 12 to surface 10 along a surface normal. It willbe appreciated that, when surfaces 10 and 12 are parallel to each other,a normal to one of the surfaces will also be a normal to the othersurface and to the prevailing plane of layer 8 and cover 2. It willfurther be appreciated that, in accordance with the above definitions,propagation angle φ_(D) may take values from 0° to 180°.

According to a preferred embodiment of the present invention, lightdeflecting elements 14 are designed to result in the propagation angleφ_(D) being greater than TIR angle φ_(TIR) at the optical interfaceformed by the light input surface of layer 8 and less than 90°. Thisensures that the light deflection by elements 14 will not prevent lightfrom reaching the light output surface yet providing for a substantiallight deviation from the original propagation path and enabling TIR atthe light input surface of layer 8. By using the above notations for therefractive indices, a preferred propagation angle φ_(D) of lightdeflected by light deflecting elements 14 may thus be expressed by thefollowing relationship: arcsin(n₂/n₁)<φ_(D)<90°.

In FIG. 1, light deflecting elements 14 are exemplified by high aspectratio prismatic cavities formed in broad surface 10. Each of the highaspect ratio prismatic cavities may be characterized by two generallyplanar and symmetrically disposed faces located between surfaces 10 and12 and inclined at an angle with respect to both surfaces. In at leastsome embodiments of the present invention and in the context ofdescribing a surface microstructure element, such as, for example, asurface cavity having a prismatic or conical shape, the term “highaspect ratio” is meant to mean a geometric configuration of themicrostructure element, in a cross-section, where the height or depth ofthe microstructure element is approximately equal or greater than itsbase at the surface. This term also includes the case when the height ofthe microstructure element is much greater than the base thuscorresponding to a deep drawn cavity or a hole with almost verticalwalls.

According to an embodiment of the present invention illustrated in FIG.1, an individual light deflecting elements 14 can be viewed as anysuitable localized interruption or alteration of the otherwise smoothsurface 10 that alters the optical interface properties of the surfacein such a way that a fan of rays entering layer 8 through any element 14will have a different angular distribution in at least one dimensionwithin layer 8 compared to the case when the same fan of rays crossessurface 10 elsewhere through spacing areas between elements 14. In anaspect of this invention, light deflecting elements 14 suppress thenormal Snell's law refraction generally characterizing the broad surface10. It will be appreciated that light deflecting elements 14 alter thesurface properties only within their active apertures while the rest ofthe surface area remains unaffected.

It will be appreciated that the cross-sectional view of FIG. 1 canrepresent different basic configurations and structures of optical cover2. By way of example, one such structure may have a linear or lenticulargeometry as it can be visually represented by the extrusion of the abovecross-section in the direction perpendicular to the drawing or by arevolution of the cross-section around a vertical axis disposed in theplane of the drawing.

FIG. 2 through FIG. 5 illustrate various configurations of lightdeflecting elements 14 which may be represented by a V-shapedcross-section. Particularly, FIG. 2 shows a deep and elongated V-groovewhich may extend all the way through surface 10 or its substantialportion. FIG. 3 shows a relatively short V-groove or a notch which canstill have an elongated shape or, alternatively, it can have identicalor similar longitudinal and transversal dimensions. FIG. 4 shows apyramidal shape of the cavity representing an individual lightdeflecting element 14. FIG. 5 shows a cone-shape cavity of lightdeflecting element 14. While straight cross-sectional profiles of thewalls formed by the surface cavities have been illustrated, it should beunderstood that any curved and/or segmented profiles may also be used toform the respective elements 14.

By way of example and not limitation, FIG. 6 and FIG. 7 illustratevariations of optical cover 2 having a cross-section shown in FIG. 1 andwhere light deflecting elements 14 have a linear configuration. In FIG.6, a rectangular sheet of layer 8 employs an array of high aspect ratiolenticular prismatic grooves formed in surface 10. Each prismatic grooverepresents an individual light deflecting element 14. Elements 14 arespaced apart from each other and are alternating with smooth spacingareas of surface 10. In FIG. 7, the lenticular prismatic grooves ofelements 15 have an annular geometry and are formed in layer 8 which hasa round shape resulting in an axisymmetrical configuration of opticalcover 2 with an axis of symmetry 50.

FIG. 8 and FIG. 9 further illustrate possible variations of opticalcover 2 having a cross-section shown in FIG. 1 and where surfacecavities of light deflecting elements 14 are discrete surface relieffeatures formed in surface 10 and distributed along the width and lengthof rectangularly shaped layer 8. Each deflecting element 14 representsan interruption in otherwise smooth and planar surface 10 and locallyalters light bending properties of the surface according to theprinciples described above. In FIG. 8, light deflecting elements 14 areformed by a two-dimensional array of high-aspect-ratio conical cavitiesdistributed according to a predetermined pattern across surface 10 oflayer 8. Each conical cavity may have a round or elongated/ellipticalaperture. In a further example shown in FIG. 9, light deflectingelements 14 are represented by prismatic or pyramidal cavities eachhaving a rectangular aperture.

It should be understood that deflecting elements 14 formed by a discretesurface microstructural features, such as V-shaped notches or cuts, maybe arranged in groups or any suitable patterns. By way of example,V-notches may be positioned adjacent to each other in one dimensionforming one or more parallel bands extending along the length or widthof surface 10 where the notches can be made either parallel orperpendicular to the bands.

Optical cover may further comprise a light collector array exemplifiedby a planar lens array 6 in FIG. 10. Lens array 6 is formed by an arrayof convex micro lenses 18 arranged on a common transparent substrate.Lens array 6 is positioned so that its side covered with lenses 18 isfacing the intended source of light and the opposite side or surface isdisposed adjacent to surface 10 of layer 8 with a small gap. Thesheetform and dimensions of planar lens array 6 are selected to matchthat of layer 8. A sandwich of lens array 6 and layer 8 can thus form atransparent layered sheet structure where lens array 6 represents afront sheet and layer 8 represents a back sheet with its surface 10being designated as the light input surface and surface 12 beingdesignated as the light output surface.

It should be understood that the thickness of both lens array 6 andlayer 8 may be varied in a broad range including the thicknesses moretypical for a plate, sheet, or film which, in turn, will determine thesuitable fabrication techniques, materials, physical properties, feeland appearance of optical cover 2. In the embodiment of FIG. 10, a smallair gap is provided between layers formed by lens array 6 and layer 8 tooptically separate the layers from each other by providing a steppeddrop in refractive index outwardly from layer 8 at surface 10.

The number and disposition of individual lenses 18 in lens array 6 areselected to match those of light deflecting elements 14 in layer 8 sothat there is a one-to-one relationship between lenses 18 and elements14. More particularly, each light deflecting element 14 is preferablyaligned with respect to the optical axis of the respective lens 18.Furthermore, the optical and dimensional parameters of lenses 18 areselected so that each light deflecting element 14 is disposed at or nearthe focal area or focus of the respective lens 18. As a practicalconsideration, the focal length of lenses 18 is selected to beapproximately equal or slightly longer than the thickness of lens array6 so that each lens 18 is designed to have a focus located outside ofthe lens array itself, preferably at a small pre-determined distancefrom the lens array. Among the factors that will determine the preferredfocal distance are the thickness of the air gap between lens array 6 andlayer 8 and the size of the cavities forming elements 14.

Accordingly, when positioned with one side representing the entranceaperture perpendicular to the incident beam, lens array 6 provides aplurality of foci on the opposite side, the foci being spaced apart fromeach other in accordance with the spacing of individual lenses in thelens array. With the lens array being planar and individual lenseshaving an identical optical configuration, the plurality of foci ofindividual lenses 18 provides a common focal plane disposed at a smallpredetermined distance from lens array 6. The entrance aperture of eachlight deflecting element 14 is selected to be substantially smaller thanthat of the respective lens 18 and to have the size approximately equalor slightly larger than the focal area of the lens.

For the purpose of illustration of this invention and from the practicalstandpoint, the terms “focal area” or “focus” of an individual lens 18of focusing lens array 6 should be understood broadly and generallyrefers to an area within the envelope of the focused beam which saidlens may form when exposed to an incident beam of light, where said areahas a cross section substantially smaller than the cross section ofrespective lens 18. Accordingly, the focal area may include areas at arelatively small distance from the “ideal” focus of the lens 18 andwhere the focused beam can be convergent (before focus) or divergent(after focus). The term “effective focal length” or “effective focaldistance” can be defined as the distance from the vertex of lens to itsfocus.

Each pair of light deflecting elements 14 and lenses 18 is thus forminga combined optical element that we hereinafter generally associate withthe term “opticule”. In the context of the present invention andreferring to arrays of optical micro-components, we define the term“opticule” as an elementary combination of a larger-aperture primaryfocusing optical component and an associated smaller-aperture secondaryoptical component disposed in the primary's focus and designed tofurther redirect or redistribute light collected by the primarycomponent. However, this term should be understood loosely and shouldnot be interpreted as limiting the scope of the present invention in anyway.

FIG. 11 shows an alternative disposition of lens array 6 with respect tolayer 8 where lens array 6 is positioned adjacent to surface 12 which isopposing to surface 10 having light deflecting elements 14 and wheresurface 10 is designated as the light output surface and surface 12 isdesignated as the light input surface of layer 8. In this case, thefocal length of each lens 18 should be adjusted compared to theembodiment of FIG. 10 to accommodate the thickness of layer 8 and theincreased distance between lenses 18 and light deflecting elements 14.Similarly, each pair of lens 18 and deflecting element 14 forms anopticule which may operate independently from the other opticules.

Lens array 6 can be formed, for example, by an array of cylindrical orpoint-focus lenses depending on the configuration of layer 8 and lightdeflecting elements 14. Lenses 18 may have a linear, or linear-focus,geometry providing light focusing in one dimension, particularly in thecase when optical cover 2 employs a linear configuration of lightdeflecting elements 14. Alternatively, each lens 18 may have apoint-focus geometry and can be configured for focusing the incidentlight in two dimensions, particularly in the case when the array oflight deflecting elements 14 is configured in a two-dimensional patternof discrete surface relief elements. However, it should be understoodthat lenses 18 forming the lens array 6 can be made in any other desiredconfiguration which provides for concentration of the received light,including but not limited to lenticular, cylindrical, round, hexagonal,square, rectangular, linear-focus or point-focus configurations orshapes. Lenses 18 can be arranged to cover the entire area of the lensarray 6 or they can be spaced apart from each other leaving one or moreportions of the lens array void of the lenses.

It will be appreciated that the individual opticules each comprisinglight deflecting element 14 and matching lens 18 can be disposedaccording to any suitable pattern throughout the prevailing plane ofoptical cover 2 and can be packed with any desired density covering aportion or the entire light receiving aperture of optical cover 2. In anon-limiting example, a plurality of opticules can be distributed alongsurface 10 as a uniform array or they can be arbitrary grouped into twoor more arrays which may or may not overlap and which may also havedifferent suitable arrangements of the opticules within them.

According to an aspect of the present invention, FIG. 12 illustrates anexemplary configuration of lens array 6 in a lenticular configurationemploying cylindrical lenses. This configuration advantageouslycorresponds to the lenticular configuration of layer 8 shown in FIG. 6.According to a further aspect of the present invention, FIG. 13illustrates a densely packed lens array 6 which correspond to eitherconfiguration of layer 8 shown in FIG. 8 and FIG. 9. Each point focuslens 18 has a square aperture which allows to cover about 100% of thelens array surface. FIG. 14 shows an alternative hexagonal shape for theapertures of lenses 18.

FIG. 15 shows an embodiment of optical cover 2 comprising rectangularlayer 8 having light deflecting elements 14 in the form ofhigh-aspect-ratio prismatic grooves and further comprising lens array 6positioned adjacent to surface 10 of layer 8 with a small air gap. Lensarray 6 has a plurality of lenses 18 disposed in optical alignment withrespective light deflecting elements 14 so that each element 14 isdisposed in light receiving relationship with respect to a matching lens18. While only a few opticules are shown in FIG. 15 for the sake ofclarity, it should be understood that optical cover 2 may ordinarilycomprise a relatively large number of micro-scale lenses 18 and lightdeflecting elements 14. By way of example and not limitation, thethickness of the resulting structure may range from a fraction of amillimeter (a film-like configuration) up to several millimeters (asheet-like or plate-like configuration) and the width and/or length ofthe structure may range from 100 mm to 1,000 mm or more.

In FIG. 16, an embodiment of optical cover 2 is illustrated in operationwith a light harvesting device 4. Each light deflecting element 14 isconfigured in the form of a prismatic groove having a high geometricalaspect ratio in a cross-section perpendicular to the longitudinal axisof the groove. Each lens/groove pair forms an individual opticule whichis capable of inputting at least the on-axis incident light into layer 8at an angle which is more favorable for light trapping. Optical cover 2is positioned on top of light harvesting device 4 and in a directcontact with the light receiving surface of the device so that there isa good optical coupling contact between light output surface 12 anddevice 4 to provide for an unimpeded light passage from layer 8 intodevice 4. It may also be preferred that the refractive index of layer 8is less than or approximately matches that of device 4 to reduceparasitic reflections at the boundary between cover 2 and device 4. Alayer of index-matched optical adhesive or encapsulant (not shown) mayalso be provided between optical cover 2 and device 4 in order tofurther promote the optical contact and surface-to-surface adhesion.

Referring to FIG. 16, a ray 82 strikes one of the lenses 18 of lensarray 6 and is further directed to a matching light deflecting element14 by means of focusing. Ray 82 further strikes an inclined surface ofthe V-shaped groove forming the light deflecting element 14 whichcommunicates a greater bend angle to the ray with respect to a normal tosurface 10. The groove configuration and the refractive index of layer 8are so selected as to result in ray 82 deflection to a new propagationangle which is greater than TIR angle φ_(TIR) at surface 10.

Ray 82 further crosses surface 12 of layer 8 and enters light harvestingdevice 4. Since there is a good optical coupling contact between layer 8and device 4, the losses at the respective optical interface arenegligible. While ray 82 is propagating in the bulk material of lightharvesting device 4, at least a portion of its energy is absorbed andconverted to whatever useful type of energy. It will be appreciated thatsince the incidence angle of ray 82 into the layer of device 4 isincreased compared to the original normal incidence, the optical path ofthe ray through the photosensitive material is also increased resultingin enhanced absorption. Yet, when the photosensitive layer of device 4is relatively thin, a portion of ray 82 may still emerge back fromdevice 4 into layer 8, for example, after reflecting from the device'sreflective back cover by means of TIR or by means of a specularreflection. The emerging ray 82 will thus strike surface 10 from theinside of layer 8. Since the propagation angle of ray 82 with respect toa normal to surface 10 is greater than the TIR angle φ_(TIR), the ray isalmost losslessly reflected from surface 10 at the same angle by meansof TIR. Ray 82 can thus further enter device 4 where the rest of itsenergy can be absorbed. This process may continue until ray 82 iscompletely absorbed in device 4. Optical cover 2 therefore providesconvenient means for recycling light which is not fully absorbed duringits initial interaction with light harvesting device 4.

It should be understood that, a portion of ray 82 may be ejected fromcover 2 into the environment when certain conditions are met. This canoccur, for example, when ray 82 strikes any light deflecting element forthe second time. However, since the area associated with lightdeflecting elements 14 is substantially smaller than that of surface 10,the probability of ray 82 striking another light deflecting element 14before it gets substantially absorbed is fairly low, which ensures theeffectiveness of light trapping and useful conversion. Ray 82 may alsorandomly obtain less-than-TIR propagation angles and exit from opticalcover 2 due to scattering within layer 8 or device 4 due to variousreasons such as optical imperfections of the material, surfaceroughness, etc. However, these effects may be minimized by selectingoptical materials with sufficiently good optical clarity and goodsurface quality. Additionally, the back surface of the light harvestingdevice 4 may be provided with good specular reflectivity.

It should be understood that optical cover 2 may be configured to admitlight from a broad angular range into device 4. Referring further toFIG. 16, a stray (off-axis) ray 120 strikes lens 18 of lens array 6 andis focused onto a smooth portion of surface 10 thus missing thecorresponding light deflecting element 14. Ray 120 still passes throughlayer 8 and enters device 4 where at least a portion of it can beabsorbed depending on the absorption properties and the thickness of thephotosensitive material. However, since ray 120 is has not beenadditionally bent by light deflecting element, its unabsorbed portion,if any, which can emerge from device 4 back into layer 8 will generallynot be trapped by means of TIR and may escape from cover 2 into theenvironment.

In FIG. 17, an embodiment of optical cover 2 is further illustrated withmore individual on-axis rays being shown. Each opticule acts anindependent optical system injecting light into cover 2 and lightharvesting device 4 at a greater bend angle compared to the unalteredsurface 10. This results in slant propagation angles for most light raysin device 4 thus increasing the optical path through the light sensitivematerial and increasing the useful absorption. Additionally, the lighttrapping further enhances light absorption due to the TIR mechanism asexplained above.

FIG. 18 shows the operation of an alternative configuration of opticalcover 2 in which layer 8 is flipped upside down so that surface 10 isfacing device 4 and surface 12 is facing lens array 6. Surface 10 istherefore designated to operate as the light output surface and surface12 becomes the light input surface of cover 2. As noted above with thereference to FIG. 11, the focal lengths of lenses 18 and/or thethickness of the respective layers should be selected accordingly toensure that light deflecting elements 14 are positioned in the focalplane of lens array 6.

Referring to FIG. 18, a parallel beam of light, such as that emanated bya distant light source or the sun, strikes the entrance aperture of lensarray 6. Lenses 18 focus the respective portions of the beam onto theplurality of light deflecting elements 14 which disperse the focusedrays within layer 8 at greater angles with respect to the normal to theprevailing plane of the layer 8. The angular distribution of thedispersed light is such that at least a substantial part of the lightrays obtains propagation angles allowing for TIR at the light inputsurface 12. The light which is injected into light harvesting device 4becomes trapped within the sandwich structure formed by layer 8 andphoto absorbing layer of device 4. The optical path and usefulabsorption of light by device 4 is thus increased due to both the highbend angles and light trapping due to TIR.

FIG. 19 shows a more specific illustrative example of using cover 2 witha light harvesting device such as a photovoltaic panel or one or morephotovoltaic cells for collecting the sunlight. As it will be seen fromthe below description, optical cover 2 can be effectively used forcollecting both the direct and diffuse light. Also, while photovoltaiclight harvesting elements are shown by way of example, it will beappreciated that the same principles can be applied for light collectorsor detectors employing any other type of electronic light harvestingdevice.

In FIG. 19, optical cover 2 comprises transparent layer 8 and lens array6. Layer 8 has opposing broad surfaces 10 and 12 and is configured foran unimpeded light passage in a broad range of incidence angles withrespect to a normal to surface 10 which is designated as the light inputsurface. Light deflecting elements 14 are formed in surface 10 byhigh-aspect-ratio prismatic or conical grooves or cavities having aV-shaped cross-section. Light harvesting device 4 comprises one or morephotovoltaic cells 44 each having a photo sensitive layer 30, multiplefront contacts 36 and a metallic back contact layer 38. A customarylayer of optically transparent encapsulant 42, such as EVA or silicone,can be provided for protecting and insulating the photovoltaic cells 44.Device 4 can also further employ a back cover 40 for protecting thephotovoltaic components from the environment and/or for furtherenhancing the light trapping in the device.

In operation, a near-normal ray 82 is focused by lens 18 onto therespective light deflecting element 14 and is further directed throughsurface 12 into device 4 where it enters layer 30 of photovoltaic cell44. In a non-limiting aspect, ray 82 can exemplify the direct sunlightor a beam of light from any other distant radiant energy source. Therefractive faces of light deflecting elements 14 are inclined withrespect to surface 10 so as to result in deflecting light rays andcommunicating them propagation angles greater than the TIR angle atsurface 10. Particularly, at least one wall of the V-groove or cavity ofelement 14 is inclined at such an angle that the propagation angle φ_(D)of ray 82 with respect to a prevailing direction 70 of light propagationthrough optical cover 2 is less than 90° and greater than the TIR angleat surface 10 of layer 8.

Layer 30 at least partially absorbs the energy of ray 82 and the rest ofray 82 is reflected from back contact 38. The reflected portion of ray82 escapes device 4 where it enters cover 2 and eventually strikessurface 10 from the inside of said layer. Ray 82 subsequently undergoesTIR at surface 10 and is directed back into photovoltaic cell 44 wherethe remaining energy of ray 82 is absorbed by layer 30.

Another on-axis ray 84 is similarly injected by a different opticule (apair of lens 18 and deflecting element 14) into cover 2. In anon-limiting illustrative aspect of the invention, ray 84 can bereflected by the surface of photovoltaic cell 44. This can occur for anumber of reasons, for example, due to the Fresnel reflection at theencapsulant/cell interface. In a more particular example, photovoltaiccell 44 can employ crystalline Si material, which refractive index isvery high (about 3.5 at 0.55 μm wavelength) compared to the commonoptical materials such as glass or PMMA and the Fresnel reflection fromits surface can be substantial. The reflected ray 84 exits lightharvesting device 4 and enters optical cover 2 where it strikes surface10. Again, the stepped drop in refractive index between layer 8 and theoutside medium provides for TIR from surface 10 at the given incidenceangle which ensures that 84 losslessly reflects from surface 10 and isdirected back into device 4 where it is absorbed by thephotosensitive/photoabsorptive layer 30 of the adjacent cell 44.

A yet another ray 86 entering light harvesting device 4 via opticalcover 2 and bent by light deflecting element 14 to a greater-than-TIRangle strikes a contact finger 36 of photovoltaic cell 44. As shown inFIG. 19, ray 86 is reflected back into cover 2 without interacting withthe photosensitive material 30. Without cover 2, ray 86 would haveescaped into the environment without a chance of being absorbed.However, due to the structure of cover 2, ray 86 is kept within thesystem by means of TIR from surface 10 and is further directed towardphotovoltaic cell 44. In view of the above illustrated examples, it willbe appreciated that optical cover 2 acts as a light trappingsuperstructure on top of photovoltaic cells and allows for recycling thephotons thus enhancing their absorption and conversion into electricity.

Referring further to FIG. 19, a stray light ray 88 enters optical cover2 at a skew angle. Ray 88 may exemplify a diffuse ambient light or anoff-axis light beam. Ray 88 interacts with lens 18 but can miss therespective light deflecting element 14 when the incidence angle isgreater than the acceptance angle of the opticule. Since both lens array6 and layer 8 are essentially transparent to the incident light, opticalcover 2 still transmits ray 88 down to photovoltaic cell 44 where it canbe either fully or partially absorbed. Additionally, each lightdeflecting element 14 may be configured so that when an off-axis raystrikes its refractive facets, it is still directed downward to lightharvesting device 4. It will be appreciated that, if ray 88 is scatteredanywhere between surface 10 of layer 8 and back cover 40 of lightharvesting device 4, it can still be trapped by cover 2 if the resultingscattering angle with respect to a normal to surface 10 becomes lessthan the TIR angle. Thus, optical cover 2 allows for harvesting theambient or diffuse light unlike optical concentrators which can onlycollect the direct light and usually send the off-axis rays away fromthe target.

In FIG. 20, an embodiment of optical cover 2 is illustrated whereoptical layer 8 is flipped upside down compared to FIG. 19 and wheresurface 12 is the light input surface and surface 10 is the light outputsurface of the optical layer. In this geometry of light incidence ontolight deflecting elements 14, the sloped faces of the respectivecavities forming light deflecting elements 14 are now redirecting therespective rays by means of TIR rather than by refraction. Due to thehigh aspect ratio of the cavities forming elements 14 and the resultingskew incidence angles, TIR can be easily realized at the interfacebetween the refractive material of layer 8 and the outside medium thatfills the cavities.

Optical cover 2 of FIG. 20 also employs an optional cladding layer 22disposed between lens array 6 and transparent layer 8. Cladding layer 22replaces the air gap of the above examples. Layer 22 should be made froma material having a lower refractive index compared to layer 8 in orderto provide for TIR at surface 10 at the sufficiently high deflectionangles. Suitable cladding materials may include low refractive indexmonomers, polymers, fluoropolymers, low-n optical adhesives, thin films,and other materials commonly used for cladding in optical waveguides,lighting panels or photovoltaic cells/panels.

The use of TIR for deflecting light by deflecting elements 14advantageously allows for obtaining larger bend angles, if needed. Thiscan ensure that light injected into light harvesting device 4 remainstrapped by the light input surface 12 despite the refractive index ofcladding layer 22 being ordinarily greater than that of the outsidemedium (e.g., air).

In operation, similarly to rays 82, 84, and 86 of FIG. 19, on-axis rays92, 94 and 96 are trapped by optical cover 2 and are more efficientlyabsorbed by photovoltaic cells 44 of light harvesting device 4, while astray (off-axis) ray 98 is simply transmitted to the photovoltaic cellswith high optical efficiency with or without light trapping.Particularly, as illustrated by example of ray 94, said ray strikes aTIR wall of the cavity forming an individual element 14 and is deflectedat propagation angle φ_(D) with respect to prevailing direction 70 oflight propagation through optical cover 2 so that φ_(D) is less than 90°and greater than the TIR angle at surface 12 of layer 8.

In the above illustrated examples, metallic contact fingers 36 ofphotovoltaic cell 44 may be replaced by a transparent electro-conductinglayer that will form the front electrodes of the solar cell. Theconducting layer can be made from any conventional transparentconducting material. Particularly, transparent conducting films (TCFs)conventionally used for photovoltaic applications can be employed. TCFcan be fabricated from inorganic and/or organic materials. An example ofinorganic films is a layer of transparent conducting oxide (TCO).Suitable materials for the TCO include, but are not limited to,aluminum-doped zinc oxide (AZO), boron-doped zinc oxide, fluorine dopedtin oxide (FTO), indium tin oxide (ITO), indium molybdenum oxide (IMO),indium zinc oxide (IZO) and tantalum oxide. The TCO layer can bedeposited by any suitable process, such as chemical vapor deposition(CVD) or physical vapor deposition (PVD). The thickness of conductinglayer can be fairly small, typically up to about a few thousandnanometers.

FIG. 21 illustrates an embodiment of optical cover 2 when it is appliedfor trapping light in light harvesting device 4 exemplified by aliquid-carrying photoreactor which can be of any suitable type. One suchuseful type of the photoreactor can be utilized for water and wastewaterdetoxification or disinfection by using sunlight or an artificialultraviolet lamp in homogeneous systems employing oxidants (ozone,hydrogen peroxide) and far ultraviolet radiation (<280 nm) or inheterogeneous photocatalytic systems that combine near ultravioletradiation (320 to 390 nm range) with light-activated oxidation catalystssuch as titanium dioxide. Optical cover 2 can be used to efficientlycapture and trap radiation from the UV source so that it can be morefully absorbed thus further enhancing the photo-reaction and watertreatment efficiency.

Another useful type of the photoreactor may employ organic or inorganicphotochemical synthesis of various materials or compounds in aqueoussolution or other liquids by using conventional light sources orsunlight. Similarly, since this process generally involves chemicalreactions that proceed with the absorption of light, optical cover 2 canbe used to increase the optical path through the photo-active materialand improve the light absorption and system efficiency. Since differentphotochemical reactions may require illumination by different portionsof electromagnetic spectrum, the material of optical cover 2 may beselected according to the application-specific spectral bands to ensurethat it is transparent to the desired wavelengths. Further suitableexamples of useful photoreactors include photobioreactors for algaegrowth or the like where the absorption by a thinner layer of activesubstance can be beneficial for reducing the system cost or enhancingthe process efficiency.

In FIG. 21, optical cover 2 employs transparent layer 8 having aplurality of light deflecting elements 14 and lens array 6 having amatching plurality of lenses 18. Light input surface 10 and light outputsurface 12 of layer 8 are substantially transparent and configured foran unimpeded light passage. Light deflecting elements 14 are formed byhigh aspect ratio cavities which can have various two-dimensional orthree-dimensional shapes, as explained above. By way of an example andnot limitation, light deflecting elements 14 can be formed by deep andnarrow elongated grooves in surface 10 in which case lens array 6 may beformed by a lenticular lens array.

Light harvesting device 4 can have a planar configuration of thephotoreactor and employ a front transparent wall 52 and a rear wall 56.However, it should be understood that the photoreactor can have anyother conventional configuration, such as the tubular shape. An aqueoussolution 54 contains light absorbing agents 58 (which can be, forexample, impurities to be treated or algae to be grown, etc.) and ispumped through the photoreactor body confined between walls 52 and 56.

In operation, light ray 102 is focused by an individual lens 18 onto therespective light deflecting element 14 formed in surface 10 where it isfurther bent to a greater than the TIR angle and is directed furthertoward surface 12 of layer 8. Surface 12 transmits ray 102 further intolight harvesting device 4 where the ray begins to interact with lightabsorbing solution 54. If ray 102 is not fully absorbed in a single passfrom wall 52 to wall 56 of the photoreactor, it is reflected by wall 56while maintaining the propagation angle with respect to the surfacenormal. Wall 56 can be made transparent and contacting the outside airby its external surface 80, in which case ray 102 can reflect by meansof TIR from surface 80. Alternatively, surface 80 or the inner surfaceof wall 56 can be mirrored to provide for a specular reflectivity. Ray102 reflected from wall 56 propagates back into cover 2 where it isreflected from surface 10 by means of TIR. This process may continue asray 102 remains confined within the light harvesting device until it isfully absorbed. Accordingly, rays 104 and 106 are trapped by cover 2 inthe light harvesting device 4 in a similar manner and are absorbed morefully than in the case of one or two passes of light through thephotoabsorptive layer without additional ray bending and trapping. Astray ray 108 which is not passing through any light deflecting element14 is still efficiently transmitted by optical cover 2 and can interactwith the light absorbing medium of device 4 albeit with a reducedabsorption efficiency compared to the rays which are properly trapped.

In FIG. 22, an embodiment of the present invention is illustrated inwhich optical cover 2 comprises transparent layer 8 having rectangularcavities formed in transparent surface 10. Lens array 6 is providedwhich employs a plurality or lenses 18 and is positioned adjacent tosurface 10 with a small gap. Thin cladding layer 22 with a lowrefractive index is provided between lens array 6 and layer 8. Lensarray 6 further comprises V-shaped (in a cross-section) extensions 24distributed according to the same pattern as lenses 18 along the surfacewhich is opposing the surface in which lenses 18 are formed. Eachextension 24 has a transversal size substantially smaller than that oflenses 18 and is positioned so that it at least partially protrudes intoone of the cavities of light deflecting elements 14. It is preferredthat the plurality of extensions 24 is disposed in or in the immediateproximity to the focal plane of lens array 6.

FIG. 22 also schematically illustrates the operation of optical cover 2of this embodiment. Ray 112 enters lens array 6 where it is directed, bymeans of focusing, by lens 18 to the respective extension 24 disposed atthe opposing surface of the array. Ray 112 is refracted by extension 24so that it receives a more slanted propagation angle within layer 8. Ray112 is further refracted by the transparent face of light deflectingelement 14 and is bent to an even greater angle with respect to thesurface normal. With such a two-stage bending, ray 112 can propagatewithin layer 8 by bouncing from both surfaces 10 and 12 by means of TIRin which case optical cover 2 can act as a waveguide and transport lightsome distances along the layer 8 with a minimal loss. In FIG. 22, theindividual opticules are formed by the combination of lens 18, extension24 and light deflecting element 14 disposed along a common optical axis.

The operation of embodiment shown In FIG. 22 is further explained belowin more detail with reference to FIG. 23. A ray 114 collected by anindividual lens 18 (not shown in FIG. 23) of lens array 6 entersextension 24 where it strikes a face 26 at point 140 beneath the planeof surface 10 of layer 8. Face 26 is inclined at a sharp angle withrespect to a normal 60 to surface 10 which is also the normal to theprevailing plane of optical cover 2. A refractive index n₂ of lens array6 is greater than that of surrounding air n₄≈1 which may normally fillthe cavity of light deflecting element 14. Ray 114 undergoes refractionat face 26 and further strikes the vertical wall of the cavity forminglight deflecting element 14. It will be appreciated that a portion ofray 114 can be reflected from face 26 due to the Fresnel reflection,although the energy of the reflected ray can be substantially less thanthat of the refracted ray up to the incidence angles approaching a TIRangle at face 26.

The refracted portion of ray 114 (indicated as a ray segment 116)undergoes a further refraction at the vertical wall of light deflectingelement 14 and enters the medium of layer 8 at an angle 154 with respectto normal 60. Layer 8 has a refractive index n₁ which is also greaterthan that of air so that ray 114 bends further away from normal 60. Thereflected portion of ray 114 (indicated as a ray segment 118) passesthrough an opposing face 28 of extension 24 and subsequently enterslayer 8 at an angle 118 to normal 60, undergoing refraction each time itpasses through a boundary between different optical media. The focallength of lenses 18 and the slope of faces 26 and 28 are selected toresult in angles 154 and 118 being greater than the critical TIR angleφ_(TIR) at surface 10. Furthermore, in order to facilitate TIR, arefractive index n₃ of cladding layer 22 is selected to be sufficientlylow to permit for TIR in a wide range of incidence angles.

It should be understood that light deflecting elements 14 may compriseany suitable optical features or devices that alter the lightpropagation through surface 10 in the desired manner. By way of exampleand not limitation, light deflecting elements 14 may be selected fromthe group of surface features consisting essentially of planar mirrors,curved mirrors, mirror arrays, prisms, prism arrays, one or morereflective or refractive surfaces, diffraction gratings, holograms,light diffusing or scattering elements, and so forth. A yet furtherexample of a useful light deflecting element 14 can be a matte-finishtextured area in surface 10 having the dimensions approximating those ofthe focal area of the respective lens 18. For cylindrical lenses 18,light deflecting elements 14 may be formed in surface 10 by narrowstrips of light-scattering textured areas each disposed in therespective lens focus. Alternatively, elements 14 may be formed bydepositing light-scattering substance in the proscribed locations ofsurface 10.

When light deflecting elements 14 incorporate prismatic groovedstructures or other surface relief micro-structures, these can befabricated using a technique for direct material removal includingmechanical scribing, laser scribing, engraving, micromachining, etching,grinding, embossing, imprinting from a master mold, photolithography,and a plurality of other known methods and combinations thereof forstructuring optical materials. In addition, the faces of prismaticgrooved structures may be optionally polished to obtain any desiredlevel of surface smoothness. Layer 8 may be configured to incorporateembedded microstructures, for example, by means of casting, embossing,extrusion, injection molding, compression molding, or similar processesand combinations of molding and machining processes thereof.

Alternatively, layer 8 can incorporate an additional layer oftransparent material, such as a plastic film or thin transparent plate,attached to face 10 and the light deflecting elements 14 can be formedin that layer. Various mechanisms, including optical lithography, may beused to create the required pattern in a light-sensitive chemical photoresist by exposing it to light (typically UV) either using a projectedimage or an optical mask with the subsequent selective removal ofunwanted parts of the thin film or the bulk of a substrate. In a furtheralternative, the transparent material can be overmolded onto surface 10in the respective areas and prismatic grooved structures can be formedin the overmold. By way of a yet further non-limiting example, anegative replica of the grooves may be formed by scribing, diamondcutting/machining, laser micromachining, ion beam etching, chemicaletching, or similar techniques followed by imprinting of it in theovermold.

FIG. 26A through FIG. 26F illustrate, in a cross-section, differentexemplary variations of microstructured features that may be used toform individual light deflecting elements 14.

FIG. 26A shows a skewed V-shaped notch or undercut made in surface 10 oflayer 8. The notch has a sloped face 62 which reflects an incident ray124 by means of TIR and deflects it from the original propagation pathtoward a more slanted angle with respect to the prevailing plane oflayer 8. The slope of face 62 is selected to result in the propagationangle of the deflected ray 124 being greater than the TIR angle atsurface 10. This ensures that any specular or TIR reflection of ray 124from the light harvesting structure that may be placed under opticalcover 2 back will not cause ray decoupling from the system and that thedesired light trapping will occur. Additionally, as described in theabove examples, the deflection of ray 124 further away from a normal tothe surface plane results in the increase of the optical path length ofthe ray through a photoabsorbtive layer due to the skew incidence, whichenhances the absorption efficiency and utility of the light harvestingdevice.

In FIG. 26B, deflecting element 14 is exemplified by an undercut havinga funnel shape formed by curved walls. Such undercut may be formed, forexample, by material ablation by a laser beam. In the case of layer 8being made from acrylic, a CO₂ laser with the operating wavelength ofabout 10 microns may be used to selectively ablate the surface materialand produce a profile similar to that of FIG. 26B. A funnel shape ofelement 14 may naturally occur during laser ablation which may alsoprovide for the sufficiently smooth, polished walls of the undercut. Thewalls may also be polished in a subsequent process which may involvethermal annealing, flame polishing, laser beam heat polishing, etc.Accordingly, ray 124 is directed toward surface 12 with deflection bymeans of TIR at face 62. The deflection angle should be sufficient toprovide for TIR at surface 10 and yet allow ray 124 to enter theunderlying light harvesting device 4 (not shown) which may be coupled tosurface 12 using a layer of index-matched optical anhesive orencapsulant.

In FIG. 26C, the undercut forming light deflecting element 14 hasparallel walls and a similar operation involving TIR from face 62. InFIG. 26D, light deflecting element 14 is formed by a discontinuity inlayer 8 and may also represent a through cut in layer 8 which can bemade by a variety of conventional means. The sloped face 62 extendingall the way between surface 10 and the opposing surface 12 reflects ray124 by means of TIR and deflects the ray at a greater-than-TIR anglewith respect to a normal to surface 10.

FIG. 26E illustrates in undercut or notch of element 14 formed in theopposing surface 12. Surface 12 is the light output surface in theillustrated case. Similarly, the slope of face 62 is sufficient todeflect ray 124 at a greater angle allowing for an increased propagationpath length and also for the light trapping effect due to meeting theconditions of TIR at surface 10.

In the example illustrated in FIG. 26F, light deflecting element 14 isformed by multiple corrugations of surface 12. Each corrugation has asloped face 62 which reflects by means of TIR and deflects ray 124 at agreater angle with respect to the surface normal. The operation of lightdeflecting element 14 of FIG. 26F may also involve the refraction oflight on the adjacent corrugations. Therefore, the slope angles of thecorrugations should be selected accordingly to provide for the bendangles sufficient for satisfying the TIR condition at surface 10.

This invention is not limited to employing light deflecting elements 14that are formed in or associated with a broad external surface of layer8. Various optical features, such as voids redirecting light by means ofTIR and/or refraction, suitable for light deflecting elements 14 may beformed in the bulk of layer 8 in any desired location between surfaces10 and 12. One suitable method of forming light deflecting elements 14in an intermediate location between surfaces 10 and 12 may includemaking a first planar sheet of transparent material having V-grooves inone of its broad surfaces and attaching a second transparent sheet ontop of the microstructured surface of the first sheet.

In different variations of the present invention, lens array 6 maycomprise any desired optical structures distributed over its frontalsurface and adapted for collecting, concentrating or collimating theimpinging light. Any known light focusing structure which collects theenergy from a larger area and focuses it to a smaller focal area can beused to form the individual focusing features of lens array 6. By way ofexample and not limitation, lenses 18 can be spherical or aspherical,imaging or non-imaging, and may also be selected from the group ofoptical elements consisting essentially of Fresnel lenses, TIR lenses,gradient index lenses, diffraction lenses, lens arrays, mirrors, Fresnelmirrors, mirror arrays and the like.

A convenient way of forming lens array 6 is by providing a transparentlayer having a large array of spherical imaging lenses 18 on one of itssurfaces. Lenses 18 may be fabricated using any conventional method suchas replication, embossing, molding, micro-machining, grinding, chemicaletching, beam etching and the like. The individual lenses 18 can beintegrated with lens array 6 and preferably comprise the same materialas the body of the array. Alternatively, lenses 18 can be disposed on atransparent substrate plate and fabricated of the same or a differentmaterial than the substrate plate. Individual lenses 18 may also beconfigured as separate pieces and attached to the substrate plate.Suitable materials include but are not limited to optical glass,polymethyl methacrylate (PMMA), silicone, polycarbonate, polystyrene,polyolefin, and any optically clear resin which is obtainable bypolymerization and curing of various compositions and other methodsdirected at creating a sufficiently optically transparent structure. Theplacement of lenses 18 in lens array 6 can be according to any suitablespatial metric and by any desired means. For example, lenses 18 can bespaced apart, contacting each other or overlapping and can be positionedin any desired pattern in the array.

In accordance with this invention, it is preferred that an effectivefocal length of each lens 18 is substantially shorter than thelongitudinal or frontal dimensions of optical cover 2 in order toachieve better compactness. For the purpose of this invention, the term“effective focal length” should be understood broadly and it alsoincludes the cases when the effective focal length of can changedepending on the optical properties of the material filling up the spacebetween lens 18 and the focal area. In other words, the location of thefocal area may be different, thus resulting in a different effectivefocal length, when a different material separates lens 18 and its focalarea. By way of example, for the same geometrical parameters of a lensforming an individual lens 18, its effective focal length can be greaterin high refractive index material (e.g., glass, silicon or PMMA) than inthe air due to the difference in refractive indexes.

It should be understood that optical cover 2 or any of its layers can bemade to any size and can also be conveniently manufactured throughreplication from a continuous roll or web of transparent polymericsubstrate material, such as PMMA, polycarbonate, polyester or the like.By way of example and not limitation, the patterns of microstructures orsurface relief features including lenses 18, cavities of lightdeflecting elements 14 and extensions 24, if any, can be engraved ontorolls or plates and then transferred to the substrate by means ofextrusion, casting and/or embossing. As illustrated in FIG. 24, variousseparate layers of optical cover 2, such as, for example, lens array 6,cladding layer 22 and transparent layer 8 can subsequently be laminatedon each other by a roller 224 resulting in a monolithic structure. Itwill be appreciated that in case of a lenticular configuration of lenses18 and light deflecting elements 14, the lamination can be done in thedirection of either parallel to the lenses 18 and elements 14 or in aperpendicular direction. As illustrated in FIG. 25, the fabricatedoptical cover 2 may have a form of a flexible sheet or film and can bestored or supplied in rolls. Furthermore, it may be bent to any suitableshape, such as, for example, a cylindrical shape, depending on theapplication.

Any of the surfaces employed in optical cover 2, especially thosecontacting with air, may be provided with a layer of anti-reflectivecoating in order to reduce the Fresnel reflections when the lightrefracts through the surface and improve the light transmission of thesystem. Alternatively, or in addition to this, an anti-reflective layercan be embedded at any suitable part of cover 2, e.g. between its layersto further promote the transmissivity and overall system efficiency.Common anti-reflective coatings such as TiO₂ deposited by AtmosphericPressure Chemical Vapor Deposition (APCVD) and Si₃N₄ deposited by PlasmaEnhanced Chemical Vapor Deposition (PECVD) may be used, for example.

According to the present invention, it may be preferred thatphotoabsorptive layer, or layers, if more than one, of light harvestingdevice 4 is relatively thin in order to reduce the intake of expensivelight absorbing materials. The photoabsorptive layer can be made so thinthat it absorbs only a small portion of the incident light in a singlepath. For example, the photoabsorptive layer thickness may be selectedso that 10% or less incident light can be absorbed in a single passthrough light harvesting device 4. However, due to the light trappingfunction of optical cover 2, the rest of the light can be absorbedthrough multiple passages of light through device 4 as well as throughincreasing the light path through the photoabsorptive layer(s) for eachpass.

Further details of operation of optical cover 2 shown in the drawingfigures as well as its possible variations will be apparent from theforegoing description of preferred embodiments. Although the descriptionabove contains many details, these should not be construed as limitingthe scope of the invention but as merely providing illustrations of someof the presently preferred embodiments of this invention. Therefore, itwill be appreciated that the scope of the present invention fullyencompasses other embodiments which may become obvious to those skilledin the art, and that the scope of the present invention is accordinglyto be limited by nothing other than the appended claims, in whichreference to an element in the singular is not intended to mean “one andonly one” unless explicitly so stated, but rather “one or more.” Allstructural, chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present invention, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

1. An optical cover for use atop a light harvesting device, comprising:a layer of optically transmissive material including a plurality oflight deflecting means each configured to alter the transversalpropagation direction of light incident onto said light harvestingdevice and communicate said light a generally greater propagation anglewith respect to a surface normal than a pre-defined critical angle;wherein the cumulative aperture of said light deflecting means is lessthan the area of said layer.
 2. An optical cover as recited in claim 1,wherein said predefined critical angle is an angle of a total internalreflection characterizing at least one surface of said layer.
 3. Anoptical cover as recited in claim 1, wherein each of said lightdeflecting means comprises at least one reflective surface inclined atan angle to a surface of said layer.
 4. An optical cover as recited inclaim 1, wherein each of said light deflecting means comprises at leastone refractive surface inclined at an angle to a surface of said layer.5. An optical cover as recited in claim 1, wherein each of said lightdeflecting means comprises a surface configured to reflect by means of atotal internal reflection.
 6. An optical cover as recited in claim 1,wherein each of said light deflecting means comprises a surface relieffeature selected from the group consisting of prismatic grooves,prismatic cavities, pyramidal cavities, and conical cavities.
 7. Anoptical cover as recited in claim 1, wherein each of said lightdeflecting means is selected from the group consisting of planarmirrors, curved mirrors, mirror arrays, prisms, prism arrays, one ormore reflective or refractive surfaces, blind holes, through holes,undercuts, notches, surface discontinuities, discontinuities in saidlayer, surface corrugations, diffraction gratings, holograms, surfacetexture, light diffusing elements, and light scattering elements.
 8. Anoptical cover as recited in claim 1, wherein each of said lightdeflecting means has an elongated shape and is aligned generallyparallel to a reference line.
 9. An optical cover as recited in claim 1,wherein said plurality of light deflecting means is arranged in acylindrical configuration.
 10. An optical cover as recited in claim 1,wherein said plurality of light deflecting means is arranged in anaxisymmetrical configuration.
 11. An optical cover as recited in claim1, further comprising a plurality of light collecting elements opticallycoupled to said light deflecting means so that each pair of anindividual light collecting element and an individual light deflectingmeans forms an individual opticule configured as a two-stage opticalsystem in which said light collecting element collects light from itsinput aperture and focuses said light onto said deflecting means havinga generally smaller aperture and configured to redirect the focused beamat a different angle compared to the incidence angle.
 12. An opticalcover as recited in claim 1, further comprising a micro-lens arraydisposed in optical communication with said plurality of lightdeflecting means.
 13. An optical cover as recited in claim 1, furthercomprising a micro-lens array disposed in optical communication withsaid plurality of light deflecting means, wherein said lens arrayincludes micro-lenses in either one of the cylindrical or point focusconfiguration.
 14. An optical cover as recited in claim 1, furthercomprising one or more optical cladding layers having a lesserrefractive index than said layer of optically transmissive material. 15.An optical cover as recited in claim 1, wherein said light harvestingdevice comprises a layer of photovoltaic material.
 16. An optical coveras recited in claim 1, wherein said light harvesting device comprises alayer of photovoltaic material and a layer of electroconductivetransparent material.
 17. An optical cover as recited in claim 1,wherein said light harvesting device comprises a layer of photovoltaicmaterial and a grid of electroconductive contacts.
 18. An optical coveras recited in claim 1, wherein said light harvesting device comprises areflective surface.
 19. An optical cover as recited in claim 1, whereinsaid light harvesting device comprises one or more devices selected fromthe group consisting of solar cells or panels, radiation detectors,light absorbers, photo-chemical reactors and photo-bioreactors.
 20. Alight harvesting device having a layered structure, comprising: aphotoabsorptive layer configured to absorb incident light, and a layerof optically transmissive material having a broad-area surface andconfigured to transmit said light into said photosensitive layer andtrap said light within said light harvesting device by means of at leasta total internal reflection from said surface.